In recent years, as speeds of networks have increased and capacities of hard disks have been expanded, a system has come out which stores digital images recorded according to MPEG or the like, in a hard disk, and transmits the stored digital images to a receiver via a network. This video network system employs a high-speed network based on IEEE1394. A storage device including a hard disk is connected via this network with a reproduction apparatus which receives data to reproduce video images.
IEEE1394 has an isochronous transfer function that ensures the amount of data which are transferred during a prescribed time period. Because the isochronous transfer ensures the amount of data transferred during the prescribed time period, this is suitable for transmission of video or audio which requires a real time property, and mounted on a large number of STBs of BS digital broadcast in Japan or digital VCR apparatuses.
IEEE1394 further has an asynchronous transfer function for transferring data or commands. This asynchronous transfer is of a mode in which the right to use a bus is obtained to transmit data when desiring to transmit data, whereby the amount of data transferred during a prescribed time period is not ensured.
At present, detailed commands are defined in IEEE1394 by DVB (Digital Video Broadcasting Project), IEEE1394 T.A. (Trade Association), and the like. Particularly according to IEEE1394 T.A., formats of MPEG transport streams to be transmitted on an IEEE1394 network, commands for controlling digital VCRs by using disk storages, and the like are determined. These commands are transferred in the asynchronous transfer mode.
However, according to IEEE1394, even when packets are outputted in the order of the asynchronous transfer and the isochronous transfer, there is no guarantee that the packets are transmitted on the network in the order of the asynchronous transfer and the isochronous transfer. More specifically, even when packets are outputted in the order of the asynchronous transfer and the isochronous transfer to a packet transmitting apparatus according to IEEE1394, the packets are sometimes transmitted in the order of the isochronous transfer and the asynchronous transfer on the IEEE1394 network.
On the other hand, when reversely reproduced images are generated with MPEG pictures by using B pictures (bidirectionally predictive coded pictures) or P pictures (inter-frame forward predictive coded pictures), display of the decoded images should be switched for each picture.
FIG. 3 illustrates a structure of an MPEG decoder using B pictures and P pictures. An MPEG image input 3a to be decoded is inputted to an MPEG decoder 31 which performs decoding that is compliant with syntaxes according to MPEG. A reference picture memory 32 is a memory that stores reference pictures which are necessary to decode MPEG images. In this case, reference pictures at the decoding of MPEG images are stored in the reference picture memory 32. The MPEG images decoded by the MPEG decoder 31 are stored in a frame memory 33, and the images in the frame memory 33 are displayed on a monitor 34.
A method of reverse playback using P and B pictures will be described. FIGS. 4(a) to 4(c) show a method of forward playback using P and B pictures. At the forward playback, when images are to be displayed in the order of B0, B1, I1, B2, P3, B4, I5, B6, and P7, pictures are inputted to the MPEG decoder 31 in the order of I1, B0, P3, B2, I5, B4, P7, and B6. Then, the MPEG decoder 31 stores reference pictures which are necessary to decode B pictures, in the reference picture memory 32 according to the order shown in FIG. 4(c). Here, the pictures denoted by “I” are I pictures, which can be decoded without requiring other reference pictures to obtain video. The pictures denoted by “P” are P pictures, which require the forward prediction with referring to an immediately preceding I or P picture to be decoded. The pictures denoted by “B” are B pictures, which, otherwise specified, require the forward prediction with referring to an immediately preceding I or P picture and the backward prediction with referring to an immediately subsequent I or P picture, to be decoded.
The pictures decoded by the MPEG decoder 31 are stored in the frame memory 33 in the order of B0, I1, B2, P3, B4, I5, B6, and P7, and displayed on the monitor 34 in the same order. At this time, a switch 36 is always in a state for passing the pictures.
FIGS. 5(a) to 5(e) shows the method of reverse playback using P and B pictures. The description is given of the method for reversely reproducing the pictures shown in FIG. 4 in the order of B4, P3, B2, and I1. In order to decode B4 picture, reference pictures P3 and I5 are required. However, to generate the reference picture P3, a further preceding I1 picture is required. That is, four pictures I1, P3, I5, and B4 are required to decode the picture B4. Similarly, pictures I1 and P3 are required to decode picture P3, and pictures I1, P3 and B2 are required to decode picture B2. I1 picture does not require other pictures to be decoded.
When the picture B4 is to be decoded now, the pictures I1, P3 and I5 should be decoded, in addition to the picture B4. The decoded pictures I1, P3, and I5 are stored in the reference picture memory 32, but it is unfavorable that these pictures are stored in the frame memory 33. Thus, the switch 36 is controlled by a display control unit 35 shown in FIG. 3 for outputting only the picture B4 to the frame memory 33 to be displayed. At this time, the display control unit 35 should generate commands 5B for instructing to perform decoding in the MPEG decoder 31 but not to display an obtained picture, and commands 5A for instructing to perform decoding in the MPEG decoder 31 and also performing image display, as shown in FIG. 5(d), thereby controlling the switch 36 so as to control the storage of pictures into the frame memory 33. The same processing is performed also for other pictures. Here, in the case of playback other than the reverse playback, which does not use B and P pictures, generated commands are all 5B.
As described above, when MPEG images are to be reversely reproduced, the function of performing control so as to decode pictures but not to display decoded images is added in units of pictures, thereby to generate images reversely reproduced using P and B pictures.
FIG. 2 illustrates a prior art image transmitting apparatus. In this figure, reference numeral 21 denotes a transmission apparatus. Numeral 22 denotes a storage device, which is a hard disk in this example, in which MPEG transport streams are recorded in an arbitrary format. Numeral 23 denotes an IEEE1394 transmission/receiving means, and numeral 24 denotes a storage device control means.
Numeral 25 denotes a reproduction apparatus, which comprises an IEEE transmission/receiving means 26, a command generation means 27, and an image reproduction means 28. In this figure, MPEG images are recorded in the storage device 22 in the form of MPEG transport streams. The command generation means 27 in the reproduction apparatus 25 generates a reproduction command 2a for reproducing images from an MPEG transport stream which is stored in the storage device 22, and transmits the generated command 2a to the IEEE1394 transmission/receiving means 26. The reproduction command 2a is compliant with a command of IEEE1394 T.A. (Trade Association), which indicates a state of the playback, such as playback, fast forward, reverse playback, stop of playback, and the like, but does not control reproduction of each picture.
The IEEE1394 transmission/receiving means 26 converts the reproduction command 2a into an asynchronous packet on the IEEE1394, and transmits the packet via the IEEE1394 network 2b to the IEEE1394 transmission/receiving means 23. The IEEE1394 transmission/receiving means 23 extracts the reproduction command 2a from the transmitted asynchronous packet, and transmits the extracted command 2a to the storage device control means 24. The storage device control means 24 reads an MPEG transport stream which is recorded in the storage device 22, corresponding to the reproduction command 2a, and transmits the readout MPEG transport stream 6a to the IEEE1394 transmission/receiving means 23.
The IEEE1394 transmission/receiving means 23 converts the MPEG transport stream 6a that is read out from the storage device 22 into an isochronous packet, and transmits the packet through the IEEE1394 network 2b. The transmitted isochronous packet is converted into an MPEG transport stream by the IEEE1394 transmission/receiving means 26, and transmitted to the image reproduction means 28. The components as shown in FIG. 3 other than the display control unit 35 are included in the image reproduction means 28. Since a decoding control signal 3b outputted from the display control unit 35 in FIG. 3 is generated in synchronized with a picture which is read out of the storage device 22, this signal is stored in the storage device control means 24.
FIG. 6 illustrates a structure of the IEEE1394 transmission/receiving means 23 and 26. The reproduction command 2a is converted into an asynchronous transfer packet by an asynchronous transfer means 61, and transmitted/received via the IEEE1394 network 2b through an IEEE1394 link layer 63 and an IEEE1394 physical layer 64. An MPEG transport stream 6a is converted into an IEEE1394 isochronous packet by an isochronous transfer means 62, and transmitted/received via the IEEE1394 network 2b through the IEEE1394 link layer 63 and the IEEE1394 physical layer 64. The method for converting the MPEG transport stream and the method for transmitting/receiving the packets by means of the IEEE1394 transmission/receiving means are determined in IEEE1394. The IEEE1394 link layer 63 and the IEEE1394 physical layer 64 are standardized by IEEE1394.
FIG. 7 shows a structure of an MPEG transport stream. In FIG. 7, reference numeral 72 denotes a bit stream that is called an elementary stream in which information such as video or audio is described. As images, MPEG video compressed bit streams are contained in many cases. This bit stream syntax is pursuant to a format that is defined by ISO/IEC 13818-2. The elementary stream 72 sometimes contains Sequence_header, in which information about the method for compressing images, identification of Standard Definition images and High Definition images, structures of pictures, and output intervals between pictures are described. A stream which comprises the elementary stream 72 and a PES header 71 that is defined by ISO/IEC 13818-1 is called a PES stream 70.
The PES stream 70 is divided into an arbitrary size, then w a header defined by ISO/IEC 13818-1 is added to obtain a fixed packet of 188-byte length, and various information is further added thereto to obtain multiple channels, resulting in an MPEG transport stream.
Here, it is assumed that an MPEG image refers to an MPEG transport stream defined by ISO/IEC 13818-1, which has elementary streams that are defined by ISO/IEC 13818-2 or ISO/IEC 11172-2 as pictures.
When the images stored in the storage device are subjected to forward reproduction, stop of reproduction, and reverse reproduction of only I pictures in the conventional structure as shown in FIG. 2, the reproduction command 2a is transmitted or received in the asynchronous transfer mode and then the isochronous transfer is resumed, as shown in FIGS. 2, 3, and 4. Even when the isochronous transfer is performed after it is confirmed that the transmission or receiving according to the asynchronous transfer is completed, images can be reproduced without problems.
On the other hand, when the reverse playback using P and B pictures is performed, a decoding control signal should be generated for each picture. As this decoding control signal depends on a structure of a GOP (Group Of Picture), this signal is contained in the storage device control means 24 in FIG. 2. Accordingly, the decoding control signal 3b that is generated by the display control unit 35 in the storage device control means 24 must be transmitted to the image reproduction means 28 in the reproduction apparatus 25 via the IEEE1394 network 2b. Since the decoding control signal 3b generated by the display control unit 35 is synchronized with an image which is read out from the storage device 22 and further this is a command signal, this signal should be transmitted in the asynchronous transfer mode through the IEEE1394 network 2b. 
To be more specific, an MPEG picture that is transferred in the isochronous transfer mode and the decoding control signal 3b generated by the display control unit 35, which is transmitted in the asynchronous transfer mode, should be transmitted in synchronized with each other on the IEEE1394 network 2b. 
When an MPEG picture that is transferred according to the isochronous transfer is transmitted to the isochronous transfer means 62 after an asynchronous packet of the decoding control signal 3b generated by the display control unit 35 is inputted to the asynchronous transfer means 61 in the IEEE1394 transmission/receiving means 23, there are some cases where the IEEE1394 link layer 63 does not perform the asynchronous transfer of a previously inputted packet but performs the isochronous transfer of a later inputted picture. This is because the transmission order of the asynchronous transfer and the isochronous transfer is not ensured in the IEEE1394 standards.
Even if the transfer is performed in the order of asynchronous transfer and isochronous transfer, there may be the possibility that the transmission order is inverted after the receiving. After the completion of asynchronous transfer is checked, i.e., when the asynchronous transfer is performed while stopping the isochronous transfer in the meantime and thereafter MPEG pictures are transmitted by the isochronous transfer means 62, the transfer rate may be adversely rate-determined to the asynchronous transfer rate, and The band for the isochronous transfer may not be ensured, because the transmission band of the asynchronous transfer is not ensured. The commands for reproduction or stop do not require to consider the transmission bands before and after the commands, but the control signal for each picture should ensure also the transmission bands before and after the command.
That is, in the conventional structure, the decoding control signal 3b that is synchronized with a picture cannot be transmitted or received to perform reverse reproduction of MPEG images.
The conventional reversely reproduced image generation apparatus has the aforementioned structure and, accordingly, when an apparatus for generating a reversely reproduced image via an IEEE1394 network is connected with an apparatus for decoding the reversely reproduced image, an MPEG picture that is transferred in the isochronous transfer mode is not synchronized with a control signal that is transferred in the asynchronous transfer mode, because the transfer order of the asynchronous transfer and the isochronous transfer is not ensured, whereby the reversely reproduced image cannot be decoded.